1. Field of the Invention
The present invention relates to a method for growing epitaxial diamond and more particularly to a method for growing epitaxial diamond on a diamond substrate after depositing a metallic layer on the diamond substrate.
2. Description of the Prior Art
Diamond possesses several unique physical and chemical properties and is considered an excellent material to apply to different technical fields. About its mechanical properties, it has extremely high hardness (Vickers hardness, Hv=50-104 GPa), low coefficient of air friction (˜0.035 to 0.15), wear-resistance, high strength (1.2×1012 Nm−2) and stiffness. As for thermal properties, diamond has superior thermal conductivity of 20 Wcm−1K−1. Regarding photoelectric properties, diamond has wide band gap (Eg=5.45 eV), high dielectric coefficients (106 V/cm), significant electron mobility (me=4500 cm2V−1s−1) and hole mobility (mp=3800 cm2V−1s−1) better than silicon, ultra high refractive index and transparency. With respect to other properties, diamond has excellent chemical stability, radiation resistance and biocompatibility. As a material which possesses desired physical and chemical properties, diamond has great potential in industry. Single crystal diamond is considered a promising material and has been researched and developed in recent years. In order to be widely used in industry, high growth rate and diamonds of larger size are desired.
The most popular method to grow diamond film is to use chemical vapor deposition (CVD). It has the characteristic of low process temperature and pressure, and enables a high growth rate of diamond film with superior purity; especially for manufacturing single crystal diamond with large area and smooth surface. Hydrogen and methane are usually supplied during the growth process. Generally, hot filament chemical vapor deposition (HFCVD) and microwave plasma chemical vapor deposition (MPCVD) are adopted. For HFCVD, a filament set above the substrate arranged inside the quartz tube is heated to 2000° C. to 2200° C. so that the carbon containing gases are dissociated to generate carbon radicals for growing diamond films. Hydrogen acts as protective gas to prevent formation of graphite. As for, MPCVD, microwave passes through the reaction chamber to dissociate low pressure gases filling inside to generate plasma. Plasma is composed of dissociated particles at high temperature such as free electrons, charged ions and neutral particles to bombard and heat substrates and thereby overcoming activation energy of chemical reactions which originally have high energy barriers. Also, it allows carbon containing gases to be dissociated at relatively lower temperature to generate carbon radicals and contributes to diamond deposition. Besides, ionized hydrogen gives higher etching rate of graphite than that of diamond. It is helpful to improve diamond quality.
Although it has been reported for years that growing high quality epitaxial diamond films is practicable, problems still remain to be solved. One main issue is that internal stress and defects formed inside diamond films during the deposition process leads to cracks. For example, intrinsic defects of substrates or impurities such as hydrogen atoms, nitrogen atoms, carbon atoms and graphite accumulated inside the diamond films form stress field. As a result, deformation and cracks occur to release stress. This phenomenon is frequently found when growing (111) epitaxial diamond and thus restricts uses of (111) diamond for electronic devices. From another point of view, growing thin films on substrates is usually accompanied with tiny deformation. In the beginning, tiny deformation can be ignored. However, if the diamond film reaches a certain thickness and diamond structure cannot be compatible with excessive deformation, stress will be released in the form of cracks.
Hence, it is necessary to develop a method to grow diamond epitaxial film with desired thickness and prevent cracks due to excessive stress.